Method for the growth of industrial crystals

ABSTRACT

A method is disclosed for producing large single crystals. According to the initial steps of this method, a plurality of single crystal wafers are crystallographically oriented to form a seed plate which is patterned. The patterned seed plate is selectively etched to expose the bare surface of the seed plate. The exposed, patterned bare surface of the seed plate is etched to form a plurality of nucleation structures. Each of the nucleation structures protrude outwardly from the underlying surface of the seed plate and provide ideal structures for the growth of large, single crystals. The resulting large, single crystals can be separated from the seed crystals by etching, physical or chemical means.

RELATED U.S. APPLICATION DATA

This is a division, of application Ser. No. 08/454,775, filed 31 May1995, now U.S. Pat. No. 5,614,019 which is a Continuation-in-Part ofpatent U.S. application Ser. No. 07/895,482, now U.S. Pat No. 5,443,032filed Jun. 8, 1992, the specifications and claims which are incorporatedby reference.

FIELD OF THE INVENTION

This invention relates to an alternative method to the method describedin parent application Ser. No. 07/895,482 for the manufacture of largesingle crystals of diamond, cubic boron nitride, silicon carbide andsimilar crystals which are difficult to manufacture in large dimensions.More particularly, the invention relates to a method for preparing seedplates for use in growing large, single crystals and a method forproducing single crystal, electronic grade diamond wafers larger thansingle crystal diamond wafers that are currently available, i.e. areasgreater than about 1 centimeter² (cm²).

BACKGROUND OF THE INVENTION

Graphite is the most stable form of carbon under normal conditions, butat pressures of approximately 600,000 atmospheres and temperaturesexceeding 1500° C., diamond is the thermodynamically stable phase.General Electric succeeded in growing synthetic diamond in themetastable region during the early 1950's by dissolving carbon in amolten transition metal at pressures in the range of about 40,000 toabout 60,000 atmospheres and temperatures in the range of about 1200 toabout 1600° C., i.e., high pressure, high temperature (HPHT) conditions,see H. T. Hall et al., U.S. Pat. No. 2,947,610. Diamond crystalsnucleate and grow from molten carbon and metal solution, typically anickel or iron solution. While most of the diamond crystals produced byHPHT conditions are under one millimeter (mm) for diamond gritapplications, several corporations have been able to produce diamondcrystals almost up to one cm. Due to the enormous difficulties withscaling up the pressure in large volumes at high temperature anddifficulties with preserving the integrity of seals, only very moderateimprovements in the size of diamond single crystals by the HPHTtechnique beyond one cm can be expected in the future.

The ideal solution for the needs of the electronic industry would behetero-epitaxial diamond on inexpensive substrates. Diamond viahetero-epitaxial chemical vapor deposition (CVD) has been reported on mmsize cubic-boron nitride (C-BN) single crystal made by HPHT; see M.Yoshikawa et al., Appl.Phys.Lett., Vol. 57, page 428 (1990). C-BN showsthe most promise as a heteroepitaxial substrate for diamond due tosimilar structure, close lattice match and high surface energy.Unfortunately, only about 1 mm size crystal of cubic BN is availablefrom HPHT techniques as of this date and the preparation of large singlecrystal cubic BN substrates from gas phase is very difficult and has notas yet been accomplished.

An interesting technique has been disclosed which utilizes an array of100 μm (0.1 mm) octahedron faceted HPHT diamond crystals fitted byspinning from a slurry into corresponding pyramidal etch pits onto asilicon wafer; see N. V. Geis et al., Proceedings of the SecondInternational Symposium on Diamond Materials, 179th Meeting of theElectrochemical Society in Washington, D.C., page 605, May 5, 1991. Inthis technique, commercially available (111)-faceted diamond seedshaving diameters of 75 to 100 μm are deposited on (100)-oriented Sisubstrates which had been patterned and etched using standardphotolithographic methods to form 90 μm square etch pits on 100 μmcenters faceted on (111)-planes. Homoepitaxial diamond is grown on thediamond seeds to form a continuous diamond film composed of a pluralityof approximately oriented small crystals. Self supporting continuousdiamond films were obtained after etching away the silicon substrate.The diamond film contains low angle grain boundaries because diamondseeds are always misoriented by a few degrees. Textured growth onsilicon or beta silicon carbide has much smaller diamond grain size onthe order of a few microns across the single crystal grain, but themisorientation of individual grains is on the same order of magnitude inboth cases.

The disadvantages of the foregoing technique are that default holesoccur in the resulting diamond film as a result of missing seed crystalsin some of the etch pits and that a slight misorientation occurs amongthe individual single crystal grains. Therefore, this prior arttechnique does not produce a crystallographically perfect singlecrystal.

The method of the present copending parent application, U.S. Ser. No.07/895,482 is directed to CVD diamond growth on a plurality of orientedsingle crystal diamond seed wafers which are patterned with a pluralityof seed holes formed in such a way that newly grown single crystaldiamond layer can be separated from a reusable diamond substrate. As aconsequence of the orientation of two or more precisely oriented singlecrystal diamond patterned structures, a large diamond single crystalseed plate is generated by epitaxial fusion.

R. A. Rudder et al., has succeeded in depositing diamonds onto aphotolithographically defined large electronic device area and hasobserved isotropic overgrowth advancing vertically and horizontally byabout the same rate; see R. A. Rudder, J. B. Posthill, G. C. Hudson, D.Malta, R. E. Thomas, R. J. Markunas, T. P. Humphreys, R. J. Nemanich,Proceedings of the Second International Conference New Diamond Scienceand Technology, Washington, D.C., page 425, Sep. 23-27, 1990. The Rudderet al. method includes the steps of depositing polycrystalline silicononto a single diamond substrate; depositing a masking layer onto the Silayer; photolithographically opening holes to allow for thehomoepitaxial diamond to nucleate; and depositing diamond onto theresulting substrate by CVD to form overgrowth extending over the Sipattern.

The preceding reference does riot teach separating the resultantdeposited diamond layer from the substrate by any means such asmechanical, physical or cutting means. The reference also fails to teachgrowing a continuous monocrystalline (i.e., a single crystal) layer ofmaterial onto a substrate and then separating that material from thesubstrate. The quality of homoepitaxially grown diamond above the holesin the masking layer on the device was claimed to be superior to thequality of underlying diamond substrate; see J. B. Posthill, R. A.Rudder, G. C. Hudson, D. P. Malta, G. C. Fountain, R. E. Thomas, R. J.Markunas, T. P. Humphreys, R. J. Nemanich, Proceedings of the SecondInternational Symposium on Diamond Materials, Proceedings Vol. 91-8 ofthe Electrochemical Society, May 5-10, 1991, page 274, Washington, D.C.

The superior quality of a laterally propagated epitaxial layer isbelieved to be due to the so called "necking effect", which isfrequently used in Bridgman or Czochralski crystal growth. Necking downthe growing crystal limits the propagation of dislocation from the seedcrystal only to the straight direction of growth, but not in lateraldirections. In the references mentioned above, R. A. Rudder and J. B.Posthill have demonstrated that the same effect of lateral overgrowthwhich is being used successfully in silicon and gallium arsenidemicroelectronics for the fabrication of three dimensional integratedcircuits can be used to fabricate three dimensional integrated circuitsin diamond microelectronics technology. The reference does not teachusing this technology to grow large monocrystalline diamond plates.

Anthony et al., U.S. Pat. No. 5,264,071, teach a chemical vapordeposition (CVD) method for making a monolithic polycrystalline (i.e.,multiple crystals) diamond sheet in which a diamond is deposited onto anon-diamond substrate which is smooth and free of surface irregularitiesto reduce physical bonding between such irregularities to the depositedpolycrystalline CVD diamond. The polycrystalline diamond sheet isseparated from the substrate by a cooling step or by dissolving theentire substrate after the deposition to facilitate the release of thediamond sheet from the polished, metallic substrate.

The electronic industry is using large semiconductor single crystalwafers ranging in size from 5 cm to 20 cm in diameter. Wafers smallerthan 5 cm in diameter currently cannot be economically produced.Therefore, for the electronic industry desires free-standing singlecrystal diamond wafers which are larger than 5 cm in diameter, so thatthe electronic industry can take advantage of the outstanding electronicproperties of diamond. In order to take full advantage of diamond'soutstanding electronic properties, diamond large free-standing singlecrystal wafers must be true single crystals which do not possess largeangle grain boundaries and should be at least comparable in quality tothe best natural diamond single crystal. Polycrystalline diamond filmswhich deviate from the ideal orientation of single crystal by as littleas 0.2 degree will display inferior electronic properties by the virtueof the presence of large angle grain boundaries. Large angle grainboundaries cause impurities to segregate on the interface between thegrains and display an undesirable concentration of structural defectsand electrical defects. These textured quasi-epitaxial diamond films orcrystalline diamond structures demonstrate inferior electricalproperties.

SUMMARY OF THE INVENTION

In contrast to the techniques disclosed in the prior art, the presentinvention is capable of generating large single crystals having highmonocrystalline perfection which can be used in electronic, optical,mechanical and other applications. This result is accomplished byproducing a highly perfect free-standing single crystal diamond waferwhich is comprised primarily of low angle grain boundaries.

According to the initial steps of this method, a plurality of substratesurfaces or single crystal wafers are crystallographically oriented toform a seed plate. The seed plate is patterned by depositing a patternedmasking layer. The patterned seed plate is selectively etched to exposethe bare surface of the seed plate. The exposed, patterned bare surfaceof the seed plate is etched to form a plurality of nucleation structuresconsisting of the seed plate material and the remaining portion of themasking layer. Each of the nucleation structures protrude outwardly fromthe underlying surface of the seed plate and provide ideal structuresfor single crystal growth. Each of the nucleation structures compriseswalls and a top surface having an area in the range of about 5 to about90% of the surface area of the original seed plate. More particularly,each nucleation structure comprises a protrusion mesa having side wallsand a geometrically patterned top surface having a surface area in therange of about 0.2 to about 60 μm².

The geometric pattern can possess any shape including a triangle orcircle, and the preferred pattern is a square. The seed plate isrecovered and any remaining masking layer material is removed from thetop surfaces of the protrusion mesas. The seed plate can then be placedinto a crystal growing reactor and epitaxial crystals are grown onto thetop surfaces of the mesas and above the spaces between the top surfacesof the mesas above the underlying substrate. A continuous layer ofmonocrystalline material is formed over a period of time to achieve athickness in the range of about 1 μm to about 1 cm. A thickness in therange of about 50 μm to about 1,000 μm is preferred.

In one embodiment of the method of the present invention, the growth ofthe monocrystalline material initially proceeds with approximately thesame growth rate on the top surfaces and the side walls of the mesas aswell as the underlying surface of the seed plate. In this embodiment, aseries of interconnecting channels are created within themonocrystalline material, i.e., between the growth of monocrystallinematerial on the side walls of the mesas and the underlying surface ofthe seed plate and below the monocrystalline material overlying thespaces between the top surfaces of the mesas. When the gaps between thetop surfaces of the mesas have been closed, a continuous layer of themonocrystalline material continues to grow on such top surfaces and onthe closed gaps between the top surfaces. There are a number of methodsto modify the seed plate so that most of the subsequent single crystalgrowth is deposited on the top surfaces of the mesas and on the closedgaps. Such methods should prevent or slow down the single crystal growthanywhere else on the side walls of the mesas and prevent or slow downsingle crystal growth on the substrate underlying these open spaces.

During the initial steps of another embodiment of the present invention,the etching step forms mesas to insure that the side walls aresubstantially straight, i.e., the side walls are substantially normal tothe horizontal plane of the seed plate. This structure is obtained byimpinging an oxidizing beam substantially normal to the horizontal planeof the substrate. The top surfaces of the resulting seed plate recoveredfrom this embodiment are deposited with a relatively thick layer of amasking or coating material having a thickness ranging from about 5 μmto about 30 μm. This can be accomplished by depositing a suitablecoating material onto the entire surface of the substrate.

Suitable coating materials include soft carbon, carbon soot, graphite,diamond-like carbon, hydrogenated diamond-like carbon, graphite,submicron-sized metal powder, aliphatic epoxy polymers, and mixturesthereof. Initially, the coating material covers the entire surfacesincluding the side walls and the top surfaces of the nucleationstructures and the underlying substrate. The resulting masked seed plateis planerized by polishing so that only the top surfaces of the mesasare uncovered, but the spaces between the mesas are protected by themasking layer. The top surfaces of bare mesas function as single crystalnuclei for the initiation of crystalline growth. After the terminationof the crystalline growth cycle, the newly grown single crystal isseparated from the diamond substrate by any etching, mechanical orchemical means.

In a variation of the above embodiment of the present invention, the topsurfaces of the seed plate are deposited with a relatively thick layerof a graphite material. This can be done by the steps of spinning anemulsion of graphite or other easily combustible coating material on thetop of the mesas and drying the emulsion. Alternatively, the step ofspinning can be replaced by the step of dipping the mesas in theemulsion. These steps of spinning or dipping and drying are repeatedseveral times until a build up of graphite exists to completely coverthe side walls and the top surfaces of the mesas and the underlyingsubstrate.

In this embodiment, the resulting completely masked seed plate is alsoplanerized by polishing to uncover the top surface of the mesas and toleave the rest of the seed plate protected by porous, dried graphite.The resulting masked seed plate, with only the top surfaces of the mesasfree of graphite, is placed on a suitable wafer holder in a CVD reactor.In this embodiment, the resultant fully grown epitaxial crystal isseparated from the seed plate by etching or burning out the remainder ofthe graphite material and etching or burning through the cross-sectionalareas of the mesas of the seed plate between the underlying surface andthe top surface of mesas. Specifically, the composite is subjected to anoxidizing atmosphere at temperatures in the range of about 650° to about1000° C. for a sufficient period of time to etch through aboutcross-sectional areas of the mesas connecting the newly grown singlecrystal with the underlying substrate until separation occurs. Thesingle crystal recovered from this process has an area that is greaterthan each of the plurality of crystal wafers that comprise the seedplate.

During the initial steps of still another embodiment of the presentinvention, the etching step forms each of the plurality of mesas in theshape of an inverted pyramid with the base of pyramid being the topsurface of the mesa and the apex being attached to the underlyingsubstrate. This is done, for example, by impinging an oxidizing beam atan angle of substantially less than 90° from the normal plane of thesingle crystal diamond substrate while the substrate is being rotated.Preferably the oxidizing beam is directed at an angle in the range ofabout 15° to about 55° from the normal, and still more preferably at anangle in the range of about 17° to about 45°.

The resulting patterned seed plate is place in a CVD reactor for thediamond growth step. Diamond deposits on the inverted pyramidal mesas insuch a way the diamond growth bridges the narrow gaps between the mesasat the tops of the mesas. In this way a continuous single crystaldiamond layer at the top of the patterned surface is formed. A networkof empty tunnels is left below this single crystal diamond growth layer.Therefore after the termination of the crystal growth cycle, the newlygrown single crystal can easily be separated from the substrate byetching in an oxidizing atmosphere due to the presence of the emptytunnels. Specifically, the composite is subjected to an oxidizingatmosphere at temperatures in the range of about 650° to about 1000° C.for a sufficient period of time to etch through the thinnestcross-sectional areas of the pyramidal mesas between the apex and thebase connecting the newly grown single crystal with the underlyingsurface of the original substrate until separation occurs.

The resulting single crystal can then be used as the seed plate in placeof the plurality of smaller crystal wafers used in the initial steps ofthe method and the remaining steps of crystal growing, separation andrecovery are repeated.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the method disclosed herein willbecome apparent from the following and more particular description ofthe preferred embodiment of the invention as illustrated in theaccompanying drawings (not to scale), in which:

FIGS. 1-6 (the proportions in these figures are distorted to moreclearly illustrate critical features) are schematic illustrations of thestep-by-step fabrication of a diamond single crystal in accordance withone embodiment of the present invention;

FIG. 2a (the proportions in this figure are distorted to more clearlyillustrate critical features) is an enlargement of one of the mesas onthe seed plate shown in FIG. 2;

FIG. 4a is a top plan view of one corner of the seed plate 12, asschematically illustrated in FIG. 4, showing the growth on the mesas ofregular diamond crystal with octagonal cross-section before the crystalsbecome fused into a single crystal;

FIG. 4b is an oblique pictorial view of the upper portion of anindividual diamond nucleus that forms on the mesas as schematicallyillustrated in FIG. 4; and

FIG. 7 and FIG. 8 (the proportions in FIG. 8 are distorted to moreclearly illustrate critical features) are schematic illustrations of twoof the essential steps in the fabrication of a large single diamondcrystal according to the method of the present invention.

FIGS. 9-15 (the proportions in these figures are distorted to moreclearly illustrate critical features) are schematic illustrations of thestep-by-step fabrication of a diamond single crystal in accordance withanother embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, planar face 10 of substrate 12, which has beenmasked with a masking layer 14 and a photoresist layer 16, is patternedwith a plurality, i.e., about 10⁵ to about 10⁸ /cm², of evenlydistributed photoresist mesas 20 by standard photolithographicalprocedures. Such procedures are used to define photoresist mesas 20, forexample, of square shape, placed in a rectangular periodic array withthe squares in the range of about 3×3 μm large separated by about 1 μmgaps. The shape of these mesas can be square, circular, hexagonal or anyother geometric shapes which allows for a higher density packing becauseit allows for a higher density of patterned features per unity ofsurface. Reactive ion etching is used to open gaps in masking layer 14by a standard conventional procedure. After etching, surface 10 ofsubstrate 12 possesses an array of masking layer mesas 22 capped withphotoresist mesas 20 separated by opened channels 26 free of maskinglayer 14 and bare substrate 12 is exposed at the bottom of channels 26.For example, channels 26 are on the order of about 1 μm wide.

The masking layer can comprise any material which is chemicallycompatible with the substrate such as metals, metal and other inorganicoxides, and ceramics. Specific examples of such materials include:silicon, tungsten, molybdenum, nickel, gold, copper, soft carbon,diamond-like carbon, hydrogenated diamond-like carbon, SiO₂, Si₃ N₄,MgO, CaO, silicon oxynitride and mixtures thereof.

FIG. 2 shows the patterned surface of substrate 12 after being subjectedto an etching step by a directional ion beam of an oxidizing gasgenerated by an ion milling machine. For example, the ion beam is set toimpinge on the plane of substrate 12 at about a 30° angle from thenormal plane of substrate 12. Substrate 12 is held by a holder whichrotates the substrate 12 at, for example, about 1 rpm. In this manner,the diamond substrate is removed from the bottoms of channels 26 in sucha way that deeper channels 30 are created in underlying substrate 12.The ion beam is also undercutting masking layer 14 mesas on top surface32 approximately at the same angle of the impinging ion beam all aroundsquare mesas 22.

For example, the depth of channels 30 after etching is approximately sixμm and the width at approximately the bottom of channels 30 in substrate12 is about three μm. The shape of the newly created mesas 36approximates an inverted tetragonal pyramid where the base 40 of thepyramid is the original masking layer square mesa 22 on the top surfaceof substrate 12 and the narrow part or apex of the pyramid approximately1 μm wide is connected to the seed plate on the opposite end of thepyramid base. FIG. 2a more clearly shows one such mesa 36. Any remainingphotoresist mesa caps 20 remaining on the top of mesas 22 are etchedaway by the action of an ion beam at the same time with substrateetching. FIG. 3 shows that masking layer mesas 22 of base 40 have beenetched away with a suitable chemical agent such as an acid or by dryetching.

As shown in FIG. 3, substrate 12 has nucleation structures 36 whichexhibit precisely oriented single crystal nuclei having a density ofnuclei of, for example, approximately 6.25×10⁶ cm⁻². Therefore,substrate 12 is ideal as a single crystal diamond seed plate for singlecrystal diamond growth by a CVD technique.

FIG. 4 shows the structure obtained following the first period ofcrystal growth on the surface of the underlying plate 12 and thenucleation structures consisting of bases 40 and the sides of pyramidalmesas 22. As the growth process continues, the epitaxial diamond crystal44 grows laterally and vertically over channels 30 and covers the entireregion above substrate 12 as shown in FIGS. 4, 4a and 5 and is separatedby channels 30 from original plate 12. Specifically, in the case ofdiamond growth on a (100) diamond substrate, the top part of one of theindividual diamond nuclei is illustrated by FIG. 4b in which each of thefacets is identified. FIG. 4b shows that the individual crystals arelimited by (100) and (111) crystallographical faces of high perfectionduring the first period of growth.

After the small diamond nuclei coalesce into a single crystal diamondlayer 50 and the diamond layer reaches the desired thickness for theapplication (4 μm to 1 mm, or more), as shown in FIG. 5, the growthprocess is stopped and single crystal diamond layer 50 is separated fromthe diamond seed plate 12 into a freestanding diamond wafer, as shown inFIG. 6, by mechanical, chemical, or other means. For example, separationcan be accomplished by etching, laser cutting, cleaving, thermal shockin a temperature gradient and other similar methods. In the case ofseparation by air at 750° C., diamond crystal mesas 36 are etchedthrough to separate crystal 50 containing upper protrusions 52 from theoriginal seed plate 12 containing the lower protrusions 54. Upper andlower protrusions 52 and 54, that remain after the etching through mesas36, can be removed by polishing.

Original single crystal diamond seed plate 12 is recovered after theseparation process is finished and can be regrown by a subsequent CVDprocess to the original thickness, repolished and reused again for thefabrication of an artificial diamond nucleation structure for anothercycle of single crystal diamond fabrication. Alternatively, the originaldiamond substrate can be repolished after separation and used again forthe fabrication of an artificial diamond single crystal nucleationstructure several times without regrowing it. However, once the diamondsubstrate gets too thin for mechanical handling, the thickness must berestored by CVD single crystal diamond growth to its original thickness.This process can be repeated many times in order to manufacture adesirable amount of new diamond single crystals.

Large diamond crystal wafers are generated by precise alignment ofseveral small crystallographically oriented diamond crystal wafers toform larger seed plates and subsequent diamond growth on the nucleationstructures until the desired size of diamond free-standing wafer isachieved. Methods for achieving precise alignment of the crystal wafersare well known in the art. FIG. 7 illustrates the assembly of four ofthese single diamond crystals, 50, 56, 57 and 58, to form square seedplate 59. However, the exact number of separate crystal-wafers that areoperably positioned to form the seed plate comprising the plurality ofcrystal wafers is not critical and will depend on the particular size ofthe final crystal product. Seed plate 59 as shown in FIG. 8 after beingpatterned masked by photolithographic procedures and etched in the samemanner as the individual crystal wafers and seed plate 59 containing thechannels 30, is placed in a crystal growing reactor to grow largeepitaxial diamond crystal 60 on seed plate 59 to form composite 62.Crystal 60 can be grown to sufficient thickness to be freestanding or toa thin crystal epitaxial layer which can be supported by an additionallayer of material for the separation.

During the embodiment of the present invention which is illustrated inFIGS. 9-15 and set forth in detail in Example 1, a masking layer is usedcomprising graphite, soft carbon, soot, diamond-like carbon, pyrolyzedpolymers and similar carbonaceous materials which are easily combusted.The epitaxially grown crystal layer is separated from the seed plate byetching the masking layer at temperatures in the range of about 250° toabout 600° C. in the presence of an oxygen-containing atmosphere andraising the temperature in the range of about 600° to about 900° C. toetch away the diamond mesas.

The commercially important feature of the present invention is thatafter the large, epitaxially grown crystal layer is separated from theseed plate, the resulting separated large crystal can be used to produceadditional single crystals. Once an inventory of seed plates of varyinglengths and widths ranging in area from about 2 to about 40 cm² has beenmanufactured, the seed plates are patterned masked and placed directlyinto the crystal growing reactor for replication of single crystals ofsubstantially the same area.

The method of the present invention produces large crystals using smallcrystal starting materials, i.e., the seed plate, such as natural andHPHT synthetic diamond crystals which are available in sizes up to about1 cm². This process can be used to produce a variety of importantcrystals which are available only in small crystal sizes which can beused as the seed crystals for the present method. Such materials includesilicon carbide and C-BN and similar crystals which are difficult tomanufacture in large dimensions.

Diamond growth for the purpose of this invention can be accomplished byany technique capable of growing diamond epitaxial layers, which is notdetrimental to the masking layer. The exact crystal growing techniquesthat are used to grow the large crystals by the present method are notcritical. The following list of techniques is illustrative, but notexhaustive: hot filament CVD (HFCVD), microwave-assisted CVD, radiofrequency plasma CVD (RFCVD), DC plasma assisted CVD, electron assistedCVD, Electron Cyclotron Resonance (ECR) plasma CVD, DC or AC arc plasmajet, and combustion flame diamond growth deposition. The followingreferences provide an illustration of the state of the art of suchtechniques: U.S. Pat. No. 4,767,608; 4,740,263; 4,734,339 and 4,707,384.

One of the most frequently used chemical compounds for diamond growth ismethane (0.1-7% vol.) with hydrogen gas making up the balance. Methanecan be replaced with a variety of other compounds containing carbon,hydrogen, halogen, and sometimes oxygen. The following hydrocarbons havebeen successfully used: methane, ethane, propane, acetylene, ethene andbenzene. The use of organic compounds such as methyl alcohol, ethylalcohol, acetone, acetic acid typically results in faster diamond growthrates. Halogens or halocarbons based mixtures with hydrocarbons oralcohols and small oxygen addition allow growth of diamond crystal atlower temperatures. A carbon monoxide mixture (e.g. 15% by volume) withhydrogen is also known to result in good quality diamond growth. Oxygenand water are sometimes added to carbon-containing mixtures in order tomodify the character of crystal growth. Inert gases are also sometimesadded to reaction mixture.

The purity of gases is critical for electronic applications in respectto species that become incorporated in the diamond crystal and areeither electrically active or result in formation of inclusions. Anexample of an undesirable impurity is nitrogen. On the other hand,electrically active impurities are sometimes deliberately added in orderto render the diamond crystal electrically conductive. Examples of suchdesirable impurities include diborane or some other boron compounds,which make diamond a p-type conductor.

Suitable temperatures for conducting the growth step can be chosen from350° to 1250° C. The preferred temperature range for HFCVD with 1%methane and 99% hydrogen is 600°-1050° C. at 30 Torr pressure in thereactor. The gas mixture pressure depends on the particular techniqueused. Typical pressure for the HFCVD technique ranges between 20-80Torr. The range of gas pressure is from low pressures, i.e. 0.5 Torrwhich is sometimes used in RFCVD, to 1 atm., which is sometimes used inthe DC arc jet technique. The flow rate of the gas mixture depends onthe specific crystal growing technique used and the size of the crystalgrowing reactor. Gas flow rates for a 5 cm diameter HFCVD reactor rangefrom 1 cm³ /min. to 100 cm³ /min., preferably in the range 5 cm³ /min.to 15 cm³ /min.

The masking layer can be deposited onto the diamond seed by a number oftechniques routinely used in the semiconductor industry such asevaporation, sputtering, ion beam deposition, CVD and the like.Patterning of the masking layer can be done by standardphotolithographical techniques routinely used for manufacturing devicesin semiconductor industry. A photolithographical mask is typicallydesigned which allows for printing a design on the photoresist, spun offon a masking layer on the top of diamond seed. The design consists ofcircles or squares or other geometrical figures with dimensions from 0.1μm-10 μm separated by distance of 0.5-20.0 μm on a rectangular,hexagonal or other grid pattern. The thickness of the masking layertypically ranges from 0.01 μm to 5 μm with the preferred range from 0.1μm-2 μm.

In the preferred embodiment of the present invention, HFCVD andmicrowave-assisted CVD reactors have been used to manufacture largesingle diamond crystals. A general description of the type of HFCVDreactors and the process conditions suitable for depositing such diamondcrystals via HFCVD are set forth in U.S. Pat. No. 5,126,206, issued Jun.30, 1992. A general description of the type of microwave-assisted CVDreactors and the process conditions suitable for depositing such diamondcrystals via microwave-assisted CVD are set forth in A. R. Badzian. T.Badzian, R. Roy, R. Messier, K. E. Spear, "Crystallization of DiamondCrystals and Films By Microwave-Assisted CVD II!", Mat. Res. Bulletin,Vol. 23, pages 531-548 (1988).

The following examples are provided to further illustrate Applicants'invention. Example 1 which follows describes another embodiment of themethod of the invention which is illustrated in FIGS. 9-15.

EXAMPLE 1 MANUFACTURE OF FREE STANDING SINGLE CRYSTAL DIAMOND USING ANUCLEATION STRUCTURE HAVING CYLINDRICAL MEAS NUCLEI

Natural II.A type diamond single crystal square substrate of (100)orientation and 8 mm×8 mm×0.25 mm dimensions was used as seed plate 70for crystal growth. The diamond seed plate was cleaned with organicsolvents by rinsing 2 min. in trichloroethane, 2 min. in acetone, 2 min.in ethyl alcohol and 5 min. in deionized water, and blown dry withnitrogen gas. Next, the wafer was placed in ion beam sputtering machinein order to deposit on its polished surface, 0.5 μm of silicon dioxide.After pulling a vacuum of 5×10⁻⁶ Torr, the diamond substrate wasprecleaned with an argon ion beam generated from an 11 cm diameterKaufman source at 500 eV and 137 mA for 1 min.

Following the precleaning step, 0.5 μm of silicon dioxide was depositedfrom a silicon dioxide source bombarded with argon ion beam generatedfrom a 5 cm diameter Kaufman source at 1000 eV and 100 mA at 1×10⁻⁴Torr. The deposition rate of silicon dioxide was 100 Å/min. This 0.5 μmsilicon dioxide layer was used as masking layer 72 in which 2 μmdiameter mesas on 4 μm centers on an rectangular grid were formed by thefollowing photolithographical procedure. After spinning positivephotoresist at 5,000 rpm for 20 sec. and 90° C. bake, the photoresistlayer 74 on the top of the substrate was exposed in the mask alignerwith the appropriate mask for 20 sec. and developed for 40 sec. in adeveloper. This step formed the mesas in photoresist. Unprotected areaof silicon dioxide masking layer were etched away by bufferedhydrofluoric acid until the diamond surface was reached (FIG. 9).Photoresist stripper at 70° C. for 10 min. was used to remove theremainder of the photoresist 74, followed by a three times repeatedrinse procedure of acetone, methanol and deionized water. The resultingpatterned diamond substrate was dried in oven at 120° C. for 15 min.

In the next step, a selected depth of the unprotected diamond surface isremoved by an oxygen anisotropic etching in a parallel plate reactiveion etching machine (RIE). After 30 min., 10 μm of diamond were removedfrom the surface by an anisotropic oxygen etching. A rectangular arrayof 10 μm tall mesas 76 were established by this procedure on the surfaceof diamond single crystal substrate. Protective cap 72 of silicon oxideon the tops of the mesas was etched away by buffered hydrofluoric acid.

Following the removal of silicon oxide from the tops of the mesas, aprotective masking layer is established in such way as to preventdiamond growth on the surface of the diamond substrate and on thediamond mesas' walls. This can be done by the number of ways and thefollowing example demonstrate just one possible procedure. In thisprocedure, several applications of graphite emulsion were build up onthe surface of the structure so that diamond mesas were completelycovered by the graphite emulsion 80 (FIG. 11). After drying thestructure in the oven at 120° C. for 30 min., the structure wasplanerized by polishing so that only the tops 82 of the diamond mesaswere uncovered, but the rest of the nucleation structures were protectedby porous, dried graphite 84 (FIG. 12). The tops of bare diamond mesasfunction as diamond single crystal nuclei for the initiation of diamondgrowth.

This artificial diamond nucleation structure was then placed on waferholder in an HFCVD reactor for diamond epitaxial growth. The details ofthe HFCVD reactor used in this example is described in U.S. Pat. No.5,160,544, issued Nov. 3, 1992, the details of which are incorporated byreference herein. After the distance between a carburized hot filamentand the artificial nucleation structure had been adjusted to about 10mm, the HFCVD reactor was refilled with helium and evacuated. A mixtureof 1% methane and 99% of hydrogen (99.999% purity) was then admittedinto the reactor at a flow rate of 10 sccm/min. and the pressure wasadjusted to 30 Torr. The temperature of the patterned diamond substratewas raised to 900° C. at a rate of 100° C./min. The temperature of hotfilament was maintained at about 2300° C. throughout the diamondepitaxial growth. Initial diamond nuclei 90 began to grow above themesas as shown in FIG. 13. Epitaxial growth of diamond was carried outfor 500 hours at a diamond growth rate of 0.51 μm/hour. At thetermination of growth cycle, helium gas was admitted to the HFCVDreactor at 30 Torr and a flow rate of 10 sccn/min. The resulting newdiamond single crystal 94 on the artificial diamond nucleation structurewas cooled at a rate of 100° C./min. to room temperature. The increasein the thickness of the new composite was measured to be 250 μm, whichwas due to the newly grown diamond single crystal above the artificialnucleation structure (FIG. 14).

After the termination of the crystal growth cycle, the undesirablediamond growth on the sides of artificial nucleation structure was cutaway with laser and the newly grown, 250 μm thick diamond single crystal94 was then separated from the diamond substrate 70 by etching in anoxidizing atmosphere. Specifically, the composite was subjected to airat 750° C. for a sufficient period of time to etch the remainder of thegraphite masking layer and etch through the 2 μm diamond mesasconnecting the newly grown single crystal diamond with the originaldiamond substrate until separation occurred (FIG. 15).

EXAMPLE 2 MANUFACTURE OF FREE STANDING SINGLE CRYSTAL DIAMOND USING ANUCLEATION STRUCTURE HAVING INVERTED PYRAMIDAL MESA NUCLEI

A synthetic high pressure high temperature diamond single crystal squaresubstrate of (100) orientation and 6 mm×6 mm×0.25 mm dimensions was usedas a seed plate for crystal growth. The diamond seed plate was cleanedwith organic solvents by rinsing 2 min. in trichloroethane, 2 min. inacetone, 2 min. in ethyl alcohol and 5 min. in deionized water, andblown dry with nitrogen gas. Next, the wafer was placed in an ion beamsputtering machine in order to deposit on its polished surface, 0.3 μmof silicon dioxide. After pulling a vacuum of 5×10⁻⁶ Torr, the diamondsubstrate was precleaned with an argon ion beam generated from an 11 cmdiameter Kaufman source at 500 eV and 137 mA for 1 min.

Following the precleaning step, 0.3 μm of silicon dioxide was depositedfrom a silicon dioxide source bombarded with argon ion beam generatedfrom a 5 cm diameter Kaufman source at 1000 eV and 100 mA at 1×10⁻⁴Torr. Deposition rate of silicon dioxide was 100 Å/min. This 0.3 μmsilicon dioxide layer was used as a masking layer in which 3 μm squaremesas on 4 μm centers on an rectangular grid were formed by thefollowing photolithographical procedure. After spinning positivephotoresist at 5,000 rpm for 20 sec. and 90° C. bake, the photoresistlayer on the top of the substrate was exposed in the mask aligner withthe appropriate mask for 20 sec. and developed for 40 sec. in adeveloper. This step formed the mesas in the photoresist. Theunprotected area of silicon dioxide masking layer was etched away byreactive ion etching until the diamond surface 10 was reached (FIG. 1).Photoresist stripper at 70° C. was used for 10 min. to remove theremainder of the photoresist, followed by a three times repeated rinseprocedure of acetone, methanol and deionized water. The resultingpatterned diamond substrate was dried in oven at 120° C. for 15 min.

In the next step, a selected depth of the unprotected diamond surfacewas removed by an oxygen anisotropic etching in a ion milling machinewith a hollow cathode ion source. The oxygen ion beam was set to impingeon the plane of the substrate under 30° from the normal. The substratewas held by a holder which rotated the substrate 1 rpm. After 120 min.,6 μm of diamond were removed from the surface by an anisotropic oxygenetching. A rectangular array of 6 μm tall mesas were established by thisprocedure on the surface of diamond single crystal substrate (FIG. 2).The oxygen ion beam removed diamond from the bottom of the open channelsso that new deeper channels are created in the underlying single crystaldiamond substrate. Silicon oxide mesas were undercut under the angle ofthe impinging ion beam all around the silicon oxide square mesas. Theshape of mesas is an inverted tetragonal pyramid where the base of thepyramid is the original square mesa on the top surface and the narrowpart of the pyramid approximately 1 mm wide is connected to the diamondplate on the opposite side of the base. Protective caps of silicon oxideon the top of the mesas were etched away by buffered hydrofluoric acid(FIG. 3).

The resulting artificial single crystal diamond containing thenucleation structures exhibited precisely oriented single crystal nucleiwith the density of nuclei 6.25×10⁶ cm⁻². It was then placed on waferholder in an HFCVD reactor for diamond epitaxial growth. The details ofthe HFCVD reactor used in this example was the same as that used inExample 1. After the distance between carburized hot filament and theartificial nucleation structure had been adjusted to about 10 mm, theHFCVD reactor was refilled with helium and evacuated. Mixture of 1%methane and 99% of hydrogen (99.999% purity) was then admitted into thereactor at a flow rate of 10 sccm/min. and the pressure was adjusted to30 Torr.

The temperature of the patterned diamond substrate was raised to 900° C.at a rate of 100° C./min. The temperature of hot filament was maintainedat about 2300° C. throughout the diamond epitaxial growth: Epitaxialgrowth of diamond was carried out for 250 hours at a diamond growth rateof 0.5 μm/hour. At the termination of growth cycle, helium gas wasadmitted to the HFCVD reactor at 30 Torr and a flow rate of 10 sccm/min.The resulting new diamond single crystal on the artificial diamondnucleation structures was cooled at a rate of 100° C./min. to roomtemperature. The increase in the thickness of the new composite wasmeasured to be 125 μm, which was due to the newly grown diamond singlecrystal above the artificial nucleation structures.

After the termination of the crystal growth cycle, the newly grown 125μm thick diamond single crystal was then laser trimmed on the edges andseparated from the diamond substrate by etching in an oxidizingatmosphere. Specifically, the composite was subjected to air at 750° C.for a sufficient period of time to thin by etching the entire structureincluding the narrow hum part of diamond pyramidal mesas between thebase and the apex until separation occurred.

Original single crystal diamond seed plate was recovered after theseparation process was finished and was regrown by CVD process to theoriginal thickness, repolished and reused again for the fabrication ofan artificial diamond nucleation structure for another cycle of singlecrystal diamond fabrication.

EXAMPLE 3 MANUFACTURE OF A CONCAVE SINGLE CRYSTAL DIAMOND MIRROR

This example describes another technique for the manufacture of aconcave single crystal diamond mirror of 5 cm diameter and 100 m focuslength and 3 mm thick. At first a diamond single crystal substrate isprepared of a convex shape with its corresponding optical power by lasermachining and polishing. The substrate is cleaned with organic solventsby rinsing 2 min. in trichloroethane, 2 min. in acetone, 2 min. in ethylalcohol and 5 min. in deionized water, and blown dry with nitrogen gas.

Next, the substrate is placed in ion beam sputtering machine in order todeposit 3000° A of gold on its polished surface. The diamond substratewith the gold layer is subjected to annealing at the temperature range800°-1000° C. for 8 hours in a closed cell. The deposition and annealingis carried out under ultrahigh vacuum. Under these conditions, the filmsplits up into monocrystalline gold particles approximately one mm largewhich are limited by facets in order to minimize the surface energy ofthe particles. The gold particles are separated by distance rangingapproximately between one and two μm. The surface between gold particlesis bare diamond suitable for etching. Individual gold particles functionas a masking layer which protects the diamond from etching.

The resulting structure is then etched for one hour and half in adirectional beam of oxygen ions in order to develop diamond mesas 6 mmtall. After etching away gold with aqua regia, a colloidal graphitemasking layer is deposited in the etched out depressions in the diamondsurface among the mesas, the surface is lightly polished to eliminatethe excess of graphite and the newly formed artificial nucleationstructure with non-planar surfaces is used for the growth of diamondsingle crystal above the nucleation structure by a CVD process.Separation of newly grown diamond single crystal is accomplished byannealing in the air at 750° C. for 5 hours in the annealing furnace.

Various other embodiments and aspects of our invention will occur tothose skilled in the art without departing from the spirit or scope ofthe invention. Having thus described the present invention, what is nowdeemed appropriate for Letter Patent is set forth in the followingappended claims.

What is claimed is:
 1. A method for preparing seed plates for singlecrystal growth comprising:(a) depositing onto a seed plate, comprising aplurality of crystallographically oriented single crystal wafers, amasking layer including a geometric pattern in the masking layer; (b)selectively etching to expose the bare surface of said seed plate; (c)etching the exposed, geometrically patterned bare surface of said seedplate to form a plurality of nucleation structures consisting of theseed plate material and the remaining portion of said masking layer, andprotruding outwardly from said seed plate; and (d) recovering a seedplate having a plurality of nucleation structures for single crystalgrowth.
 2. The method of claim 1 wherein each of said plurality ofnucleation structures comprises walls and a top surface having a surfacearea in the range of about 5 to about 90% of the surface area of theoriginal seed.
 3. The method of claim 2 wherein each of said pluralityof nucleation structures comprises a protrusion mesa having walls and atop surface having a surface area in the range of about 0.2 to about 60μm².
 4. The method of claim 3 wherein step (c) is carried out so thateach of said protrusion mesas are shaped into inverted pyramids; whereinany masking layer remaining after step (c) is removed from the topsurface of said protrusion mesas; wherein a single crystal is grown ontothe top surface of said protrusion mesas to form a continuous layer ofmonocrystalline material and to leave interconnecting channels withinsaid monocrystalline material; wherein the resulting single crystal isseparated from said seed plate; and wherein said single crystal havingan area greater than the area of each of the individual crystal wafersis recovered.
 5. The method of claim 4 wherein said single crystal has athickness in the range of about 1 μm to 1 centimeter.
 6. The method ofclaim 4 wherein said single crystal has a thickness in the range ofabout 50 μm to 1000 μm.
 7. The method of claim 2 wherein said seed platerecovered from step (d) is deposited with a coating material; whereinthe coating material is removed from the top surface of said nucleationstructures; wherein a single crystal is grown onto the surface of saidnucleation structures and the coating material to form a continuouslayer of monocrystalline material; wherein the resulting single crystalis separated from said seed plate; and wherein said single crystalhaving an area greater than the area of each of the individual crystalwafers is recovered.
 8. The method of claim 7 wherein said singlecrystal has a thickness in the range of about 1 μm to 1 centimeter. 9.The method of claim 2 wherein said seed plate comprises a plurality oforiented single crystal diamond wafers.
 10. The method of claim 9wherein said seed plate recovered from step (d) is deposited with acoating material; wherein the coating material is removed from the topsurface of said nucleation structures; wherein diamond is grown onto thesurface of said nucleation structures and the masking layer to formsingle crystal diamond; wherein the resulting single crystal diamond isseparated from said seed plate; and wherein said single crystal diamondhaving an area greater than the area of each of the individual singlecrystal diamond wafers is recovered.
 11. The method of claim 9 whereinstep (c) is carried out so that each of said protrusion mesas are shapedinto inverted pyramids; wherein any masking layer remaining after step(c) is removed from the top surface of said protrusion mesas; wherein adiamond is grown onto the top surface of said protrusion mesas to form acontinuous layer of single crystal diamond and to leave interconnectingchannels within said single crystal diamond; wherein the resultingsingle crystal diamond is separated from said seed plate; and whereinsaid single crystal diamond having an area greater than the area of eachof the individual crystal wafers is recovered.
 12. A method for singlecrystal growth by chemical vapor deposition comprising:(a) depositingonto a seed plate, comprising a plurality of oriented crystal wafers, amasking layer including a geometric pattern in the masking layer; (b)selectively etching to expose the bare surface of said seed plate; (c)etching the exposed, geometrically patterned bare surface of said seedplate to form a plurality of nucleation structures consisting of theseed plate material and the remaining portion of said masking layer, andprotruding outwardly from said seed plate, each of said protrusionstructures having substantially straight side walls and a top surfacehaving a surface area in the range of about 0.2 to about 60 μm² ; (d)removing the remainder of said masking layer; (e) depositing a layer ofa coating material having a thickness in the range of about 5 to about30 μm onto said seed plate; (f) planarizing the coated seed plate toexpose the top surface of said mesas; (g) placing said seed plate into achemical vapor deposition reactor; (h) growing a single crystal on thetop surface of said protrusion mesas and laterally over the coatingmaterial layer to form a continuous layer of monocrystalline material;(i) separating the resulting single crystal from said seed plate; and(j) recovering said single crystal having an area greater than the areaof each of the individual crystal wafers.
 13. The method of claim 12wherein said single crystal according to step (j) is used as said seedplate in place of the plurality of oriented crystal wafers.
 14. Themethod of claim 12 wherein said single crystal is selected from thegroup consisting of diamond, boron nitride and silicon carbide.
 15. Themethod of claim 14 wherein said single crystal is chemical vapordeposited at temperatures in the range of about 350° to about 1250° C.16. The method of claim 12 wherein said crystal wafer is selected fromthe group consisting of diamond, boron nitride and silicon carbide. 17.The method of claim 12 wherein the thickness of said epitaxially growncrystal layer is in the range of about 1 μm to about 1 centimeter. 18.The method of claim 12 wherein the thickness of said single crystal isin the range of about 50 μm to about 1000 μm.
 19. The method of claim 12wherein the composition of said masking layer comprises a materialselected from the group consisting of silicon, tungsten, molybdenum,nickel, gold, copper, soft carbon, diamond-like carbon, hydrogenateddiamond-like carbon, graphite, SiO₂, Si₃ N₄, MgO, CaO, and siliconoxynitride.
 20. The method of claim 19 wherein said coating material isselected from the group consisting of soft carbon, carbon soot,graphite, diamond-like carbon, hydrogenated diamond-like carbon,graphite, submicron-sized metal powder, aliphatic epoxy polymers andmixtures thereof.
 21. The method of claim 12 wherein said single crystalis separated from said seed plate and the resulting separated seed plateis reused in the production of larger single crystals.
 22. A method forsingle crystal growth by chemical vapor deposition comprising:(a)depositing onto a seed plate, comprising a plurality of oriented crystalwafers, a masking layer including a geometric pattern in the maskinglayer; (b) selectively etching to expose the bare surface of said seedplate; (c) etching the exposed, geometrically patterned bare surface ofsaid seed plate to form a plurality of nucleation structures consistingof the seed plate material and the remaining portion of said maskinglayer, and protruding outwardly from said seed plate, each of saidprotrusion structures in the shape of inverted pyramidal mesas havingside walls and a top surface having a surface area in the range of about0.2 to about 60 μm² ; (d) removing the remainder of said masking layer;(e) placing said seed plate into a chemical vapor deposition reactor;(f) growing a single crystal onto top surface of said inverted pyramidalmesas to form a continuous layer of monocrystalline material and toleave interconnecting channels within said monocrystalline material; (g)separating the resulting single crystal from said seed plate; and (h)recovering said single crystal having an area greater than the area ofeach of the individual crystal wafers.
 23. The method of claim 22wherein said single crystal according to step (h) is used as said seedplate in place of the plurality of oriented crystal wafers.
 24. Themethod of claim 22 wherein said single crystal is selected from thegroup consisting of diamond, boron nitride and silicon carbide.
 25. Themethod of claim 22 wherein said seed wafer is selected from the groupconsisting of diamond, boron nitride and silicon carbide.
 26. The methodof claim 22 wherein the thickness of said epitaxially grown crystallayer is in the range of about 1 μm to about 1 centimeter.
 27. Themethod of claim 22 wherein the thickness of said epitaxially growncrystal layer is in the range of about 50 μm to about 1000 μm.
 28. Themethod of claim 23 wherein said single crystal diamond is chemical vapordeposited at temperatures in the range of about 350° to about 1250° C.29. The method of claim 22 wherein the composition of said masking layercomprises a material selected from the group consisting of silicon,tungsten, molybdenum, nickel, gold, copper, soft carbon, diamond-likecarbon, hydrogenated diamond-like carbon, SiO₂, Si₃ N₄, MgO, CaO, andsilicon oxynitride.
 30. The method of claim 29 wherein said singlecrystal is separated from said seed plate and the resulting separatedseed plate is reused in the production of large single crystals.